Had my car on a dyno this weekend and discovered something happening at 5,500 rpm to redline. Once the car hit approx 5,500 rpm the boost gauge needle started to bounce from 14 to 10 psi repeatedly to redline. The HP curve on the Dyno chart reflects this. The HP curve bounces up and from 5,500 to redline. The owner of the dyno said he believes its either issues with the wasgate and/or boost controler. I had a new set of 99 spec turbos put on a few weeks ago. The boost controller is stock and operates with a PFC and nothing has been done to the stock wastegate. Any ideas of where to look?

 

"nothing has been done to the stock wastegate"

I think you have answered your own question.

 

what does that have to do with it?

can you post the dyno sheet so we can take a look.

was it a boost spike or continuously going from 10-14psi? does it happen on the street as well?

 

sounds like you need to port your wastegate, or use a exhaust restrictor plate.

 

Could be ignition breakup - that's pretty common at high RPM when you're making good power.

Dale

 

What is your afr like at 5,000rpm+?

 

Will post a scan of the Dyno graph in a few days. I did manage to make 277 rwhp and 225 lbs torque. It looked like I was well on my way to 300+ hp before this issue kicked in. I don't suspect ignition breakup as the issue. You could cleary see the intercooler couplings swell up a little and then realease as the boost bounced from 14-10 psi - almost like a heart beat. I am thinking the the ECU is detecting a boost spike, cuts on the boost controller dropping it back down to 10 psi over and over again. What steps do I need to take to prevent this?

 

Was not hooked up to a wide band, this was just a simple "Dyno-Day" with the local vette club not a tunning session.

 

What's the frequency of the oscillation?

 

very quick, drops to about 10 psi instantly and then pressure builds to aroun 13-14 psi and then drops again. Between 5,500 to redline the graph shows 8 times

 

Was is a hard or soft cut, The stock fuel cut is hard, sepecially when you reach 14 psi

 

I doubt it was fuel cut... if you hit it, you'll know it.

 

It doesn't look like boost creep. With boost creep, you'll usually see the torque continue to climb as RPMs increase because the wastegate cannot bleed off enough exhaust to maintain desired boost level.

The graph below (lower one, no x's) shows an engine with creep. The one above it shows the creep fixed.



Sonny

 

what do you mean by the boost controller is stock

 

What is the Boost duty cycle on the 2ndary on your PFC? If the duty cycle is too high you will overshoot your target boost and oscillate.

 

I will check the the secondary boost cycle today. The graph did not look like the one above with boost creep. Was much more like a wave ~~~~~~ from 5, 500 up or as books put it, an oscillation. Thanks for the help! I hope its something simple like that. books - can you explain to me what the boost cycle seetings does?

 

A simple explanation can be found in the PFC manual on page 28...Lightning in a World of Thunder also describes duty cycles.

A quick and easy test to determine if the settings are causing the oscillations is to set the 2ndary to its lowest setting...I think this is 50%.

If this eliminates the oscillations, You'll need to gradually increase the duty cycle until your target boost is reached, since the lowest duty cycle setting will take a longer time to reach your target boost

 

You mentioned that you have a stock boost controller. This needs further explanation. Did you mean the boost controller in the PFC? If so, it is NOT a reliable boost controller.

 

The boost controller is controlled by the PFC. I only meant that I did not have a Profec-B or anything like that. I set the duty cycles for the secondary turbo down from 64% to 40% to start. Same problem except that the boost maxs out at 10psi and the bounces down to 8 psi over and over again for abou 6,000 rpms and up. This is annoying.

 

Try bypassing the boost control solenoid. Hard-wire the vac line. It should limit boost to whatever the can spring is (I think it's 8 psi IIRC). That will narrow it down to being a problem with the boost control feedback loop and not some resonance in the intercooler piping, for example.

 

I had a boost problem like this, and trying to troubleshoot I blew the motor. It wound up being a bad Fuel pressure Sol.

x...

 

I had a similar symptom (with additional mods), Corksport/Chris & Derrick were able to troubleshoot the culprit. You might be able to give them a call (though Chris is no longer there IIRC). As Derrick explained to me, a vaccume hose was vice versa'd with another. Sorry for lack of info.

Laters,
Edgardo
(You could let him know I have the black FD from Seattle)

 

speculating... the PFC appears to be compensating for some small boost leak